A fuel cell can convert chemical energy to electrical energy by promoting a chemical reaction between two reactant gases.
One type of fuel cell includes a cathode flow field plate, an anode flow field plate, a membrane electrode assembly disposed between the cathode flow field plate and the anode flow field plate, and two gas diffusion layers disposed between the cathode flow field plate and the anode flow field plate. A fuel cell can also include one or more coolant flow field plates disposed adjacent the exterior of the anode flow field plate and/or the exterior of the cathode flow field plate.
Each flow field plate has an inlet region, an outlet region and open-faced channels connecting the inlet region to the outlet region and providing a way for distributing the reactant gases to the membrane electrode assembly.
The membrane electrode assembly usually includes a solid electrolyte (e.g., a proton exchange membrane, commonly abbreviated as a PEM) between a first catalyst and a second catalyst. One gas diffusion layer is between the first catalyst and the anode flow field plate, and the other gas diffusion layer is between the second catalyst and the cathode flow field plate.
During operation of the fuel cell, one of the reactant gases (the anode reactant gas) enters the anode flow field plate at the inlet region of the anode flow field plate and flows through the channels of the anode flow field plate toward the outlet region of the anode flow field plate. The other reactant gas (the cathode reactant gas) enters the cathode flow field plate at the inlet region of the cathode flow field plate and flows through the channels of the cathode flow field plate toward the cathode flow field plate outlet region.
As the anode reactant gas flows through the channels of the anode flow field plate, the anode reactant gas passes through the anode gas diffusion layer and interacts with the anode catalyst. Similarly, as the cathode reactant gas flows through the channels of the cathode flow field plate, the cathode reactant gas passes through the cathode gas diffusion layer and interacts with the cathode catalyst.
The anode catalyst interacts with the anode reactant gas to catalyze the conversion of the anode reactant gas to reaction intermediates. The reaction intermediates include ions and electrons. The cathode catalyst interacts with the cathode reactant gas and the reaction intermediates to catalyze the conversion of the cathode reactant gas to the chemical product of the fuel cell reaction.
The chemical product of the fuel cell reaction flows through a gas diffusion layer to the channels of a flow field plate (e.g., the cathode flow field plate). The chemical product then flows along the channels of the flow field plate toward the outlet region of the flow field plate.
The electrolyte provides a barrier to the flow of the electrons and reactant gases from one side of the membrane electrode assembly to the other side of the membrane electrode assembly. However, the electrolyte allows ionic reaction intermediates to flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly.
Therefore, the ionic reaction intermediates can flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly without exiting the fuel cell. In contrast, the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly by electrically connecting an external load between the anode flow field plate and the cathode flow field plate. The external load allows the electrons to flow from the anode side of the membrane electrode assembly, through the anode flow field plate, through the load and to the cathode flow field plate.
Because electrons are formed at the anode side of the membrane electrode assembly, that means the anode reactant gas undergoes oxidation during the fuel cell reaction. Because electrons are consumed at the cathode side of the membrane electrode assembly, that means the cathode reactant gas undergoes reduction during the fuel cell reaction.
For example, when hydrogen and oxygen are the reactant gases used in a fuel cell, the hydrogen flows through the anode flow field plate and undergoes oxidation. The oxygen flows through the cathode flow field plate and undergoes reduction. The specific reactions that occur in the fuel cell are represented in equations 1-3. EQU H.sub.2.fwdarw.2H.sup.+ +2e.sup.- (1) EQU 1/2O.sub.2 +2H.sup.+ +2e.sup.-.fwdarw.H.sub.2 O (2) EQU H.sub.2 +1/2O.sub.2.fwdarw.H.sub.2 O (3)
As shown in equation 1, the hydrogen forms protons (H.sup.+) and electrons. The protons flow through the electrolyte to the cathode side of the membrane electrode assembly, and the electrons flow from the anode side of the membrane electrode assembly to the cathode side of the membrane electrode assembly through the external load. As shown in equation 2, the electrons and protons react with the oxygen to form water. Equation 3 shows the overall fuel cell reaction.
In addition to forming chemical products, the fuel cell reaction produces heat. One or more coolant flow field plates are typically used to conduct the heat away from the fuel cell and prevent it from overheating.
Each coolant flow field plate has an inlet region, an outlet region and channels that provide fluid communication between the coolant flow field plate inlet region and the coolant flow field plate outlet region. A coolant (e.g., liquid de-ionized water) at a relatively low temperature enters the coolant flow field plate at the inlet region, flows through the channels of the coolant flow field plate toward the outlet region of the coolant flow field plate, and exits the coolant flow field plate at the outlet region of the coolant flow field plate. As the coolant flows through the channels of the coolant flow field plate, the coolant absorbs heat formed in the fuel cell. When the coolant exits the coolant flow field plate, the heat absorbed by the coolant is removed from the fuel cell.
To increase the electrical energy available, a plurality of fuel cells can be arranged in series to form a fuel cell stack. In a fuel cell stack, one side of a flow field plate functions as the anode flow field plate for one fuel cell while the opposite side of the flow field plate functions as the cathode flow field plate in another fuel cell. This arrangement may be referred to as a bipolar plate. The stack may also include monopolar plates such as, for example, an anode coolant flow field plate having one side that serves as an anode flow field plate and another side that serves as a coolant flow field plate. As an example, the open-faced coolant channels of an anode coolant flow field plate and a cathode coolant flow field plate may be mated to form collective coolant channels to cool the adjacent flow field plates forming fuel cells.
Typically only a portion of the fuel (e.g., reformate containing hydrogen) flowing through a fuel cell will react, so that the fuel gas exhaust from a fuel cell will generally contain some level of hydrogen. For example, the amount of hydrogen that is reacted may depend on factors including temperature, pressure, residence time, and catalyst surface area. For this reason, excess hydrogen may be sometimes fed to a fuel cell in order to increase the amount of reacting hydrogen to a level corresponding to a desired power output of the fuel cell. For example, it may be that 100 standard liters per minute (slm) of hydrogen must be reacted in a fuel cell to achieve a desired power output, but it is determined that 140 slm of hydrogen must be fed to the fuel cell to achieve this reaction of 100 slm of hydrogen. This system may be said to be running at 40% excess hydrogen at the anode inlet. In other terminology, this system may also be characterized as running at a stoichiometry of 1.4. For similar reasons, it may be desirable to supply the cathode side of the fuel cell with an excess of oxidant (e.g., air).